Although research has attempted to develop a low-cost solar cell suitable for terrestrial and space applications, progress has been slow. Silicon cells are relatively inexpensive and have a moderately high efficiency, but they tend to have lower specific powers (watts/kg) because they use bulk materials which increase the overall weight. Although suitable for many terrestrial applications, these single crystal silicon cells have diminishing utility for space missions now that it is becoming ever more critical to conserve weight. Modern satellites seek a power supply that furnishes more power at a reduced weight to allow larger payloads that consume larger amounts of energy for space, the specific power of an array should be at least 300 watts/kg. Accordingly, research has shifted to focus on inexpensive, lightweight, high efficiency cells for these space applications. For instance, research has demonstrated a graded ternary I-III-VI.sub.2 chalcopyrite semiconductor or photovoltaic transducer, as described in detail in U.S. Pat. No. Re. 31,968 and U.S. Pat. No. 4,523,051 (which are incorporated by reference into this description), that is lower cost, moderately efficient, and hardened against radiation. Efficiencies of about 10 to 11 percent AMO can now be achieved with these cells. Similarly, II-VI heterojunction solar cells, such as ZnTe-CdSe, have also been studied as shown, for example, by Gashin et al., Radiative Recombination in ZnTe-CdSe and ZnSe-CdTe Heterojunctions, Journal of Luminescence, vol. 15, 109-115 (1977), and Buch et al., Photovoltaic Properties of n-CdSe/p-ZnTe Heterojunctions, Applied Physics Letters, vol. 28, No. 10, 593-595 (May 16, 1976).
Neither the I-III-VI.sub.2 nor the II-VI cells even as thin films independently achieves the goals of weight and power demanded for space and terrestrial solar cells. Solar cells of the present invention, however, combine these I-III-VI.sub.2 and II-VI cells into a mechanically-stacked or monolithic thin film tandem to achieve a higher efficiency photovoltaic transducer that is particularly suited for space applications. The cell has surprisingly high resistance to radiation damage, is low cost due, in part, to polycrystalline structure, and is relatively lightweight due to thin film technology.